FIG. 1 shows a partially cut away view of a conventional alternating current generator, or “alternator,” such as the type used in conjunction with an internal combustion engine in a motor vehicle. The alternator shown in FIG. 1 comprises housing 37 which encloses rotor 11, stator 15, and rectifier bridge 31.
Rotor 11 comprises rotor core 43 mounted on rotor shaft 33. Pulley 35 is mounted on the end of rotor shaft 33 outside housing 37. Field winding 13 surrounds rotor core 43. Field winding is surrounded by a pair of interlocking “clawfoot” iron shells 19, 21.
Field winding 13 is connected to a pair of copper slip rings 23, 25 mounted concentrically on rotor shaft 33. Slip rings 23, 25 rotate as rotor shaft 33 rotates. Stationary carbon brushes 27, 29 are held in contact with slip rings 23, 25, respectively. In operation, an electric current is passed through field winding 13 via carbon brushes 27, 29 and slip rings 23, 25. When field winding 13 is energized, one of the interlocking iron shells becomes a magnetic “north,” and the other interlocking iron shell becomes a magnetic “south.” The interlocking nature of the shells results in a plurality of alternating north poles and south poles.
Stator 15 is a stationary steel core holding stator windings 17. The stator windings usually (but not always) consist of three individual sets of windings connected in a delta or wye configuration. Rotor 11 is rotated within stator 15, typically by application of a rotational force to pulley 35. Bearings 39, 41 are interposed between rotor shaft 33 and housing 37, and support rotor shaft 33 as it rotates. As rotor 11 rotates, the rotating alternating north and south poles induce an alternating current in stator windings 17.
Rectifier bridge 31 is electrically connected to stator windings 17. Rectifier bridge 31 converts the alternating current produced by stator windings 17 into direct current that is useable to charge a battery (not shown) and/or to supply other electric loads. In a motor vehicle application, a battery (not shown) typically is connected in parallel with rectifier bridge 31 to deliver adequate electric current to any electric loads when rotor 11 is not rotating or when rotor 11 is rotating too slowly to result in a voltage equal to the battery voltage. When the rotor 11 rotates at an increased speed, a voltage results across the battery terminals that is greater than the battery voltage, and the battery thereby is re-charged.
A typical alternator also comprises a voltage regulator (not shown in FIG. 1). A voltage regulator is an electronic circuit that senses the output voltage from the alternator. If the voltage regulator detects that the output voltage is low, it will supply additional current to field winding 13. The increased current enhances the strength of the rotor's magnetic field, thereby increasing the output voltage from the alternator. Likewise, if the voltage regulator detects that the output voltage is high, it will reduce the current supplied to field winding 13. The reduced current weakens the rotor's magnetic field, thereby decreasing the output voltage put from the alternator.
In a typical application, the alternator's rotor shaft can rotate at about 6000–9000 rpm. The rotation of the rotor shaft subjects bearings 39, 41 and carbon brushes 27, 29 to frictional wear, which degrades the performance of the rotor shaft bearings and the carbon brushes over time. Ultimately, the degraded rotor shaft bearings and/or carbon brushes can result in an unanticipated failure of the alternator, disabling the vehicle in which the alternator is installed.
A vehicle unexpectedly disabled by an alternator failure always is troublesome. However, an unexpected alternator failure is particularly problematic for trucks and other vehicles involved in transportation and logistics. A disabled vehicle may result in millions of dollars in lost productivity or products. Assembly lines can be idled if parts are not available. Perishable materials can expire. Accordingly, it is desired to provide a method and apparatus for predicting the failure of the frictional components of an alternator.